Conventionally, an electron beam emitter includes an electron emission source that can emit electrons and a vacuum chamber that contains the electron emission source. The vacuum chamber has a transmission window that can transmit electrons. The transmission window includes foil that transmits electrons and a grid that supports the foil. The grid rises in temperature as it completely absorbs electrons from the electron emission source. Thus, a cooling mechanism is provided on the periphery of the transmission window to remove the excess energy and keep the grid at an acceptable operating temperature. In summary, the electron beam emitter accelerates electrons, which are emitted from the electron emission source, in the vacuum chamber and passes the accelerated electrons through the foil of the transmission window (the e-beam window) so as to emit electron beams to the outside (atmospheric side) of the vacuum chamber.
Generally, in a typical electron beam emitter shown in FIG. 6, foil 12 absorbs a small proportion of electrons e− of electron beams 14 having reached the foil 12 and transmits the other proportion of electrons e−, whereas a grid 10 absorbs all the electrons e− of electron beams 14 having reached the grid 10(10b). The absorption of electrons e− into the transmission window 9 increases the temperatures of the grid 10 as well as the foil 12.
As the electron e− exposed surface of the grid 10b is comparable to the electron e− exposed surface of the foil 10a, the fractional foil 12 absorption versus full absorption at the grid 10 leads to low transmission efficiency of electrons e− in prior art designs.
An objective of the present invention is to provide an electron beam emitter that has high electron transmission efficiency that allows for either                (1) low temperature operation of the transmission window at the same output (high reliability and increased life) or        (2) increased current density and higher output at the same operating temperature.        
There are prior patents (e.g., U.S. Pat. No. 8,339,024) that achieve this goal by intercepting the waste beam with an additional structure, however, these concepts will result in internal high temperatures that would create undesired outgassing and additional thermal management requirements. The novelty of this invention is in eliminating the waste portion of the beam at the source and providing the most efficient solution.